A maximum voltage selection circuit is generally required in an integrated circuit. The main function of the maximum voltage selection circuit is to select and output a maximum voltage from a plurality of existing voltages to supply other circuits in the integrated circuit.
FIG. 1 is a maximum voltage selection circuit with three channels of voltages. As shown in FIG. 1, when the differences in voltage between a first input voltage VCC1, a second input voltage VCC2 and a third input voltage VCC3 are relatively large, the maximum voltage selection circuit can select a maximum voltage as an output voltage VMAX. However, since generally, neither of the first input voltage VCC1, the second input voltage VCC2 and the third input voltage VCC3 is zero volts, a gate connection of a P-type metal oxide semiconductor field effect transistor (PMOS) connected to one of the first input voltage VCC1, the second input voltage VCC2 or the third input voltage VCC3 may have inadequate driving capacity to provide the output voltage VMAX at the output of the maximum voltage selection circuit. Additionally, when the voltage differences between the first input voltage VCC1, the second input voltage VCC2 and the third input voltage VCC3 are relatively small or the voltages are the same, the PMOS devices in the maximum voltage selection circuit may not operate normally may not generate the output voltage VMAX.